The photorefractive effect involves light-induced charge redistribution in a nonlinear optical material to produce internal electric fields which by virtue of the optical nonlinearity, produce local changes in the index of refraction such that dynamic, erasable holograms are formed which diffract light. The photorefractive effect is achieved by exposing the material to an optical intensity pattern consisting of bright and dark regions, such as formed by interfering two coherent laser writing beams. Mobile charge generated in the material migrates under the influence of diffusion and drift processes to form internal space charge electric fields, i.e., a charge grating. Since the nonlinear material has an electro-optic effect, the electric field from the charge grating produces a grating in the optical index of refraction which causes light diffraction during readout. An important additional property of some photorefractive materials is asymmetric two beam coupling, which occurs when the pattern of index of refraction changes is spatially shifted from the original bright and dark optical intensity pattern. Asymmetric two beam coupling can be observed when two coherent beams are overlapped in the material and the optical power of the two transmitted beams is measured by art known techniques. Asymmetric two-beam coupling occurs if the optical power of one of the two beams decreases while the optical power in the other beam increases during grating formation. It is most advantageous if the beam which has increased in optical power realizes net power gain even with the photorefractive material absorption considered.
Inorganic crystals exhibiting the photorefractive effect are well known in the art as described in Genter and Huignard, "Photorefractive Materials and Their Applications", Vol. I and II ("Topics in Applied Physics" Vols. 61 and 62) (Springer, Berlin, Heidelberg 1988). Inorganic photorefractive crystals have been fabricated into optical articles for the transmission and control (change phase, intensity, or direction of propagation) of electromagnetic radiation. Several inorganic crystals exhibit net gain in two beam coupling such as BaTiO.sub.3, Bi.sub.12 SiO.sub.20, and Sr.sub.x Ba.sub.1-x Nb.sub.2 O.sub.6 as summarized for example by Yeh in Applied Optics, Vol. 26, p. 602, 1987. Net gain is essential to several applications such as self-phase conjugation, novelty filtering, optical limiting by beam fanning as described for example by Feinberg in Physics Today, Vol. 41, p. 46, October 1988.
However, it is technically difficult to fabricate such crystals into desired thin layered devices such as optical waveguides or multiple layer stacks. Further, it is challenging to dope crystalline materials with various different dopants to achieve desired property improvements such as increases in the speed and/or magnitude of the photorefractive effect because dopants are often excluded from the crystals during growth.
Schildkraut et al. U.S. Pat. No. 4,999,809 (issued Mar. 12, 1991) discloses polymeric materials which are described as being photorefractive, Ducharme et al., U.S. Pat. No. 5,064,264 (issued Nov. 12, 1991) discloses certain polymeric materials which are shown to be photorefractive. These polymeric materials can be fabricated into thin layered devices such as optical waveguides or multilayer stacks. Further, they can be readily doped with organic materials to improve the photorefractive effect. Generally, an external electric field is required in order to assist in the charge redistribution process. As a result of the applied field, a nonzero phase shift can occur between the index of refraction grating and the optical light pattern. At sufficiently high applied electric field, polymeric photorefractive materials have been shown to exhibit asymmetric two-beam coupling as described by Walsh and Moerner in the Journal of the Optical Society of America B: Optical Physics, Vol. 9, p. 1642 (1992), the disclosure of which is incorporated herein by reference. However, there has been no disclosure of net gain in photorefractive two-beam coupling for polymeric photorefractive materials. There still is a need in the art for a process for net two-beam coupling gain with polymeric photorefractive materials.
It is therefore an object of the present invention to provide an improved process for photorefractive two-beam coupling resulting in net gain. Other objects and advantages will be apparent from the following disclosure.